Making the Ringworld

Natural planets capture a minute fraction of the life-giving energy of their stars. Earth’s cross-sectional area facing the Sun is 0.45 billionths of the total area at its orbital distance. What can be done to reduce the wastage? Freeman Dyson originally proposed civilized beings might build an immense spherical cloud of habitats to maximize the sunlight captured. Some presentations of his idea, perhaps incorrectly, represented him as proposing a solid shell surrounding the Sun.

Given nuclear strength materials a civilization can make such immense structures, immensely bigger than planets in area – Dyson Spheres, Niven Ringworlds, Alderson Disks and Banks Orbitals, all of which more efficiently capture the energy of the central star. I’ll discuss them all in turn, but let’s look at the Niven Ringworld, so named because Larry Niven’s novel “Ringworld” (1970) first presented the concept to a wide audience in fictional form.

The basic idea is that a continuous ring around a star is rotated to provide centrifugal gravity on its inside face. The speed of the Ringworld’s spinning has to be incredibly high – a Ringworld at Earth-like insolation from a Sun-like star would be spinning at 1,438 km/s to produce Earth-like gravity. Niven uses that to good effect in his tale, but the engineering practicalities boggle the mind.

First let’s look at the strength required. Assuming the outward centrifugal force on each unit area of the Ringworld is what is stressing the structure, we can compute the Hoop Stress, s, as…

s = P.r/t

…where P is the outward pressure, r the hoop radius and t the thickness of the material. The radius is somewhat larger than Earth’s orbital radius – a Ringworld experiences day/night cycles via “Shadow Squares” which shade the ring, but orbit closer in, thus allowing 24 hour night/dark cycles. This means the heat experienced is somewhat more, on average, so there needs to be an adjustment to compensate. I estimate a distance of about 1.4 AU is optimal, thus r = 2.11E+11 metres.

A method proposed to supply the mass needed, and extend the life of the Sun, is called “Star-lifting” which would provide 1E+30 kilograms of mass to play with. Thus a Ringworld 2.11E+11 metres in radius and 1E+9 metres ribbon-width would have an areal density of ~7.5E+8 kilograms/kg^2 and experience an outward pressure of about 7.4E+9 N. That means the Ringworld material needs to be strong enough to withstand a P.r stress of 1.56E+21 per metre of its thickness. Alexander Bolonkin estimates the strength of nuclear matter to be roughly ~1.6E+32 N/m^2, thus a thickness of 2E-11 metres is enough to provide the x2 safety factor for the mass loading implied above. The Ring will definitely be strong enough. In fact it can probably be made with significantly less nuclear strength material.

If we want 100 metre thicknesses of water or soil (50/50 split in area) then the total outward pressure of that would be about ~1.96E+6 N/m^2 and would require ~4.14E+17 N/m of stress to be supported by the nuclear matter. But we have to factor in the mass of the nuclear matter as well. After a bit of algebra we can compute the nuclear matter layer is just ~5 femtometres thick, with an areal density of ~2,500 kg/m^2 (assuming density of 5E+17 kg/m^3.) Thus the total mass of the Ringworld is just 2.6E+26 kg – about twice the mass of Neptune. A lot less than the half-a-Sun we had to play with.

How much energy is required to boost it up to speed? Lots. Roughly 23,000 years worth of the Sun’s output, which is a truly immense amount. But if we use rockets to do the job, powered by fusion, then we need only about 10% of the mass of the Ring (Vex ~0.05 c.) That’s surprisingly not a big burden on a project of this scale and relatively reasonable. Of course a mass of rockets boosting such a Ring up to speed would produce a brilliant display of energy that should be visible as a massive X-ray flare… which makes one wonder just how many Red-dwarf “flare-stars” are really undergoing massive natural flares and not fine-tuning bursts from their Ringworld motors?

9 thoughts on “Making the Ringworld

  1. Hi Adam, if someone were to make a Dyson sphere, they perhaps would use a red dwarf rather than a G class star and shift its spectrum towards blue by reflecting half the light it emitted back to it.
    just an idle thought.

  2. By reflecting EM radiation back to the star, wouldn’t the shell lift the temperature of the stars surface?
    Sort of like enhancing the greenhouse effect on Earth, or wrapping tin foil around a light bulb.

  3. Oddly enough the effect would be to cause the Star to swell and redden. Astrophysicists think of weird scenarios like a star being in a reflective cavity and that IIRC is the counterintuitive result.

  4. (Reading backwards through the archives…)

    “If we want 100 metre thicknesses of water or soil”

    The thing about Niven’s version of the ring world is that it’s ridiculously wide, but the surface rock/soil is achingly thin. Hence it is a geologically dead “world”. (And in later novels he has to invent elaborate maintenance systems to deal with silt/flup.)

    But you could have a geologically living “world” if you cut the width down ten or twenty-fold, but made the rock depth more planetary (ie, 5000km, depending on the thermal properties of the Scrith.) Real continents, mountains and oceans, instead of sculpted Scrith. And no danger of meteors punching through. But still a phenomenal amount of living space. (And only 50 times the pressure-per-m^2.)

    Likewise, most Ringworlds, including Niven’s, are depicted with 1000km high walls at right-angles to hold the atmosphere. But if it’s a million kms wide, then a slight upward curve at the edges would “cup” the atmosphere. (Plus give you a bare Scrith surface above the atmosphere, easily reachable from the surface. A perfect place for hypersonic maglev transportation.)

    Similar slight surface rises could ecologically divide the Ringworld into separate (but reachable) “worlds”, with isolated atmospheres/geology/etc.

    1. Hi Paul.451

      Sorry for my tardy reply. Busy doing Icarus stuff and Dad duties.

      The mass of that much artificial planet boggles the mind. The idea of a Ringworld is to increase the surface to mass ratio compared to natural planets, not undo that effort utterly. But maintaining the surface cover will require some very tricky efforts by the designers, rather than ad hoc imaginings by SF novelists :-)

  5. I know another way to make a ringworld. Instead of scrith, we can use 1000 Jupiter masses of Tungsten, relying on the matter’s gravitational weight at an AU’s distance instead of its tensile strength. 1000 jupiter’s inward weight will about equal 1 jupiter’s outward weight at a centripetal acceleration of 1g towards the Sun the two balance out. I’ve calculated that a ringworld could be built around Alpha Centauri A with a radius of 184,375,961.81 km, it rotates with a tangential velocity of 1,344.66 km/sec and takes 10 days to make a complete rotation around the star at this velocity. Alpha Centauri B, the second star in the system can be pushed outward by the 1000 jupiter masses of infalling tungsten using gravitational flybys. tungsten is nice and compact, about 20,000 tungsten tracks each one about 25 wide and 50 miles deep seperated by intervals of empty space of 25 miles are all that’s needed to support a 1,000,000 mile wide ringworld band. The ringworld material can enclose such no rotating tracks and the enclosing surfaces can be used as excess radiators to keep the inner surface of the ringworld from overheating. So the tungsten tracks don’t rotate (1000 jupiter masses) but the inhabited surface does ( 1 jupiter mass) the two balance out.

  6. Hi Tom

    Several points:
    (1) what stops the tungsten weights from falling into the Sun?
    (2) 1,000 Jupiter masses is 1 Sun mass… you’ve blown the mass budget!
    (3) Where do you get 1,000 Jupiter masses of tungsten from anyway?

    Thanks for thinking about it, though.

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